It seems that there are versions without the plate retention clip bars, as shown in the photos on the SKCP wiki page. Maybe those are another series (like SKCO or SKCQ), or maybe they are just a variant model. It's highly unlikely that we will find out!

Alps plate spring switches from the 5576 and the P70 come in two kinds. They are the same except for the bottom half of the switch/it's mounting method. All plate springs have the prongs rotate 90 degrees versus other alps switches, so you cant just pop them into another alps pcb.

early
Earlier switches are plate mount and have a different bottom half case that is capable of clipping into a plate similar to any common alps switch. They have two prongs for soldering coming out of the bottom. These prongs are the contacts.
If you look at this imagewiki/File:IBM_23F3212_--_switch_disassembled.jpg

You see that the switch has a housing that clips into a plate. As well you can see by the leaf sandwhich there are two black plastic nubbins. The space beside the nubbins is empty.

later
The PCB mount switches have a bottom base that is not capable of clipping into a plate. It does not have any clipping mechanism on the switch. Instead there are the two lead contacts just like the plate mount, but on the other side of the switch is a retention bar that provides two more prongs for soldering to the board. So the PCB mount switches are held in place by 4 solder points. A pretty "sturdy-ish" cheap method.
In this image by sandy by the nubbins you can see the metal bar. http://sandy55.fc2web.com/keyboard/ibm_ ... sp_sw3.jpg

I have desoldered many of the later switches and they are all held in place by 4 prongs.

If you want to make a custom plate spring small board you will need to design a pcb that can hold the extra prongs. Or try to get the plate mounted switches variant